1. Field of the Invention
This invention relates to a method of continuously producing ethyl silicate. More specifically, it deals with a method of accurately controlling the flowrate of the reactants in relation to the temperature of the reaction.
2. Description of the Prior Art
Ethyl silicate, available commerically for many years, has been used in paints and as binders in the refractory and foundry industry. The most commercial form of ethyl silicate sold and used is ethyl silicate 40, which contains 40% silica.
Ethyl silicates were first prepared batchwise by adding a predetermined quantity of liquid silicon tetrachloride to an agitated, jacketed reactor and then introducing an excess of ethyl alcohol at a controlled rate. The reactor contents were chilled to prevent the undesirable volatilization of silicon tetrachloride before reaction which lowered product yields. Hydrogen chloride gas was removed by stripping with dry air or heating and stirring the mixture.
If anhydrous ethanol is used, the reaction which produced tetraethyl orthosilicate is represented below: EQU SiCl.sub.4 +4C.sub.2 H.sub.5 OH.fwdarw.Si(OC.sub.2 H.sub.5).sub.4 +4HCl
If ethanol containing some water is used, the reaction can be described as follows: EQU SiCl.sub.4 +(4-n)C.sub.2 H.sub.5 OH+n/2H.sub.2 O.fwdarw.Si(OC.sub.2 H.sub.5).sub.4-n O.sub.n/2 +4HCl
When n=1.57, ethyl polysilicate containing 40% SiO.sub.2 is produced, which, as set forth above, is termed ethyl silicate 40.
Partial ethanolysis of silicon tetrachloride can occur according to the following reactions: EQU SiCl.sub.4 +C.sub.2 H.sub.5 OH.fwdarw.(C.sub.2 H.sub.5 O)SiCl.sub.3 +HCl EQU (C.sub.2 H.sub.5 O)SiCl.sub.3 +C.sub.2 H.sub.5 OH.fwdarw.(C.sub.2 H.sub.5 O).sub.2 SiCl.sub.2 +HCl EQU (C.sub.2 H.sub.5 O).sub.2 SiCl.sub.2 +C.sub.2 H.sub.5 OH.fwdarw.(C.sub.2 H.sub.5 O).sub.3 SiCl+HCl EQU (C.sub.2 H.sub.5 O).sub.3 SiCl+C.sub.2 H.sub.5 OH.fwdarw.Si(OC.sub.2 H.sub.5).sub.4 +HCl
During the batch preparation of tetraethoxysilane (tetraethyl orthosilicate), several side reactions take place, including hydrogen chloride reacting with the ethanol to produce ethyl chloride and water and the excess silicon tetrachloride catalyzing a reaction which yields diethyl ether and water from ethanol: ##STR1##
The water produced then reacts with the silicon tetrachloride and the tetraethoxysilane product: EQU SiCl.sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4HCl EQU 2Si(OC.sub.2 H.sub.5).sub.4 +H.sub.2 O.fwdarw.(C.sub.2 H.sub.5 O).sub.3 SiOSi(OC.sub.2 H.sub.5).sub.3 +2C.sub.2 H.sub.5 OH EQU Si(OC.sub.2 H.sub.5).sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4C.sub.2 H.sub.5 OH
When an excess of silicon tetrachloride is present, it can react with ethyl orthosilicate to give polymeric products: EQU Si(OC.sub.2 H.sub.5).sub.4 +SiCl.sub.4 .fwdarw.2(C.sub.2 H.sub.5 O).sub.2 SiCl.sub.2
In order to reduce the extent of the side reactions which lower yield and consistently produce an ethyl silicate 40 of acceptable quality, the reaction is performed continuously with steady removal of "crude" ethyl silicate 40 and evolution of hydrogen chloride gas. The crude material is purified by distillation, neutralization, and filtration to remove the undesirable excess ethanol, reaction by-products, and residual hydrogen chloride.
Although both batch and continuous processes for producing ethyl silicate have been known in the past, the accuracy of maintaining the continuous production of high yields of the precise ethyl silicate, having the desired percentages of SiO.sub.2 content, has presented many problems, some of which are set forth above. There is also the problem of reliance on the accuracy of the two flow meters, one in harsh silicon tetrachloride service, plus the careful storage of the ethanol solution to prevent moisture absorption. There is, therefore, a need for a more accurate control for producing high yields of ethyl silicate having predetermined amounts of SiO.sub.2.